An insulated gate-type bipolar transistor (hereinafter referred to as IGBT) is a voltage-driven semiconductor switching device capable of a high-speed turning-off operation with relatively low voltage application, which is used widely in the power-electronics field in for example, inverters and other similar devices.
An IGBT output-type inverter device may have an overcurrent flow into the IGBT if there is an inrush current when a motor is actuated, and failures such as a load short circuit and arm short circuit occur. Hence, superior electrical characteristics are required for the IGBT to protect against a high voltage and large current input. One important electric characteristic is the capacity to withstand breakdown known as short-circuit withstand capability.
In accordance with this need for short-circuit protection, the inverter device incorporates a protection circuit to detect short-circuit failures when they occur resulting in turning the power supply off. However, this protection circuit requires 10 to 20 .mu.sec to detect the overcurrent and engage its protective function. The IGBT must not break down due to the overcurrent during this period.
Therefore, many recent high-performance IGBT modules adopt an overcurrent-protection system disposed independently of the protection circuit in the inverter device, which can detect at a high speed an overcurrent flowing into the IGBT when a short-circuit failure occurs, and which can limit the current flowing in the IGBT to be within the short-circuit withstand capability of the IGBT. The overcurrent protection system operates by means of a gate control operated based on an overcurrent detection signal before the power supply is turned off by the first protection circuit.
FIG. 6 shows a prior art IGBT independent overcurrent-protection circuit according to the over-current protection system. The circuit includes a main element 1 (IGBT), a current-detection sub-element 2 (different IGBT from the main element 1) connected in parallel to the main element 1, a current-detection resistance 3 connected in series to the sub-element 2, and a switching element (MOSFET) 4 connected to the gate-driving circuits. The main element 1 and the sub-element 2 perform on-off operations according to the voltage generated across the current-detection resistance 3.
Given such a configuration, when an overcurrent due to load short-circuit failure or the like flows into the main element 1 and the sub-element 2 and causes the voltage drop generated between both ends of the current-detection resistance 3 to exceed the threshold voltage of the switching element 4, the switching element 4 turns on to reduce the gate voltage to both the main element 1 and the current-detection sub-element 2 thus limiting the main current flowing in the IGBT main element 1. Thus, the main current flowing in the IGBT main element 1 can be suppressed to be within the short-circuit withstand capability of the IGBT element 1 by means of adequately setting the resistance of the current-detection resistance 3 and the threshold voltage of the switching element 4.
When an overcurrent-protection circuit that includes the IGBT sub-element 2 for current detection is constructed as an external, independent circuit to protect the IGBT main element 1 as described above in connection with the second protection circuit, it is technically difficult to maintain the operational characteristics of the main element 1 proportional to those of the sub-element 2. In other words, since the short-circuit phenomena in an inverter includes various modes such as an arm short circuit, series short circuit, output short circuit and ground fault, and it is anticipated that the collector-to-emitter voltage VCE applied to the IGBT element 1 to be protected will vary according to the short-circuit mode. This may cause the current ratio between the main element 1 and the sub-element 2 to vary, and may therefore vary the limited-current value if the collector-to-emitter voltage VCE varies as described above, making a stable overcurrent-protection operation difficult.
To solve the above problem, a configuration has already been proposed by the same applicant of the present invention in Japanese patent application No. 5-256197, wherein some of the cells formed integratedly on a semiconductor substrate are used as sensing cells to detect the current in the IGBT main element 1, and the emitter electrodes of the sensing cells are laid out separately from the emitter electrodes of the main cells formed on the same substrate and connected to a current-detection resistance in an overcurrent-protection circuit.
In the construction that incorporates the main cells of the IGBT and the current detection sensing cells of the IGBT on the same semiconductor substrate as described above, the gate electric potential of the sensing cell may change with a voltage drop at the current-detection resistance of the overcurrent-protection circuit which is connected to the sensing cells, depending on the position of the sensing cells and the IGBT output characteristics. The change in the gate electric potential generates a difference in the current density between the main cells and the sensing cells causing the current ratio to vary. Experimental results have shown that the change thereof in the collector-to-emitter voltage VCE due to the current ratio variation causes the limited-current values to change. In particular the limited-current values increase in a low voltage region in which the collector-to-emitter voltage VCE is low.
Moreover, if the voltage dependence of the limited-current values increases, trouble may occur in the overcurrent-protection operations if the IGBT is applied to an inverter device. Hence, it is necessary to suppress the voltage dependence of the limited-current values, keeping it as low as possible.